The present invention relates to a four-wheel drive system for driving four wheels of an automotive vehicle, and particularly to a control of driving of the wheels at the time of the occurrence of a wheel slip.
As a conventional four-wheel drive system for an automotive vehicle, there is known a system sensing a slip of driving wheels and reducing the torque of the slipping wheels to inhibit the slip, as disclosed in JP-A-2002-67723, for instance.
However, since the system disclosed in JP-A-2002-67723 is based on a technique not taking account of the fact that the coefficient of dynamic friction is lower than the coefficient of static friction. Therefore it may be that when a torque applied to the slipping wheel is reduced upon sensing of the slip, the driving force becomes insufficient, thereby deteriorating the running performance of the vehicle.